Lysophosphatidic acid (LPA, 1-acyl, 2-hydroxyl-sn-glycerol-3-phosphate) is an intermediary metabolite in all cells but is released from some cells to act as a mediator that elicits a wide variety of responses from cells/tissues. These responses include calcium mobilization, cytoskeletal rearrangements, mitogenesis and anti-apoptotic (survival) activity. For example, LPA is released by activated platelets and accumulates in serum to low micromolar levels, where it is a prominent growth factor for many cell types. LPA has also been found in ascitic fluid from ovarian cancer patients where it promotes mitogenesis of ovarian tumor cells. Interestingly, the LPA found in serum vs. ascitic fluid differs in that LPA from ascitic fluid is reportedly enriched in 2-acyl LPA species. Study of this 2-acyl LPA isoform is made difficult however by its chemical instability, i.e., the rapid migration of the acyl chain to the thermodynamically favored 1 position in an aqueous environment. Transient rises in blood pressure in rats and guinea pigs has also been documented following intravenous LPA injection. The induction of platelet aggregation and fibroblast recruitment along with its mitogenic capabilities implicate this lipid as a wound healing hormone.
LPA signals cells in part via a set of G protein-coupled receptors named LPA1, LPA2, and LPA3 (formerly Edg-2, Edg-4 and Edg-7). These receptors share 50-55% identical amino acids and cluster with five other receptors (S1P1, S1P2, S1P3, S1P4, S1P5 (formerly Edg-1, Edg-5, Edg-3, Edg-6, Edg-8) for the structurally-related lipid sphingosine 1-phosphate (S1P). LPA1 is most associated with activation of Giα pathways and is expressed in oligodendrocytes and peripheral tissues while LPA2 and LPA3 are associated most prominently with Gq/11α pathways. LPA2 mRNA is found in testis and peripheral blood leukocytes while LPA3 mRNA has been localized to prostate, testes, pancreas, kidney, and heart.
The physiologic implications of occupation of individual LPA receptors are largely unknown due in part to a lack of receptor type selective ligands. Therefore there is a need for compounds that have strong affinity and high selectivity for LPA receptors. The present invention is directed to a series of 2-substituted ethanolamide derivatives that vary in degrees of size, hydrophobicity, and stereochemistry. The parent compound of the claimed series, N-acyl ethanolamide phosphate (NAEPA) has been shown to be nearly indistinguishable from LPA in evoking platelet aggregation and GTP[γ35 S] binding at LPA1 and LPA2 containing membranes but is distinctly less active than LPA at recombinant LPA3 or in depolarizing Xenopus oocytes.
Three 2-substituted NAEPA compounds have already been reported. A 2-carboxyl-containing compound (named ‘NASPA’ for N-palmitoyl serine phosphate) has been documented to antagonize both LPA-driven platelet aggregation (Sugiura et al., 1994 Arch Biochem Biophys 311: 358-368) and oocyte depolarization (Santos et al., 2000 NYAS Meeting Report; Annals New York Academy of Sciences p 232-241) and is a partial agonist at mammalian LPA receptors. A 2-methylene hydroxy-containing compound, which is an analog of 2-acyl LPA wherein the ester is replaced by an amide, has been reported as activating recombinant LPA receptors in a stereoselective fashion while mitogenic responses and platelet aggregation did not show this stereoselectivity. Finally, a third compound (named ‘PNPA’ for N-palmitoyl-norleucinol-1-phosphate) has n-butyl located at the 2 position. This compound aggregates human platelets without regard to stereoselectivity. The present invention is directed to a series of prodrug derivatives of compounds active at LPA receptors.